1. Field of the Invention
The present invention relates to an organic light emitting diode display and, more specifically, to an organic light emitting diode display capable of improving uniformity in brightness by compensating a threshold voltage of a driving transistor.
2. Discussion of Related Art
Recently, various flat panel displays having weight and volume less than a comparable cathode ray tube display have been developed. Among these, organic light emitting diode (OLED) displays have drawn a lot of attention because of its excellent emission efficiency, brightness and viewing angle, as well as its quick response time.
The light emitting diode has a structure including an emission layer (or a thin film for emitting light) interposed between a cathode electrode and an anode electrode, and has a characteristic in which electrons and holes are injected into the emission layer and then recombined to generate an exciton that emits light when the exciton drops into a lower energy level.
In the light emitting diode, the emission layer is made of an inorganic or organic material, and can be classified as either an inorganic light emitting diode or an organic light emitting diode depending on the type of the emission layer.
FIG. 1 is a circuit diagram showing a pixel of a conventional organic light emitting diode display. Referring to FIG. 1, the pixel includes an organic light emitting diode (hereinafter, referred to as OLED), a driving transistor M2, a capacitor Cst, and a switching transistor M1. Further, a scan line Sn, a data line Dm, and a power line Vdd are connected to the pixel. The scan line Sn is formed in a row direction, and the data line Dm and the power line Vdd are formed in a column direction. Here, n is any integer between 1 and N, and m is any integer between 1 and M.
The switching transistor M1 has a source electrode connected to the data line Dm, a drain electrode connected to a first node A, and a gate electrode connected to the scan line Sn.
The driving transistor M2 has a source electrode connected to the pixel power line Vdd, a drain electrode connected to the OLED, and a gate electrode connected to the first node A. Further, the driving transistor M2 supplies a current to enable the OLED to emit light using a signal input to the gate electrode of the driving transistor M2. An amount of the current of the driving transistor M2 is controlled by a data signal applied through the switching transistor M1.
The capacitor Cst has a first electrode connected to the source electrode of the driving transistor M2, and a second electrode connected to the first node A, and retains a voltage between the source electrode of the driving transistor M2 and the gate electrode of the driving transistor M2 applied with the data signal, during a constant period.
With such a configuration, when the scan signal applied to the gate electrode of the switching transistor M1 turns on the switching transistor M1, a voltage corresponding to the data signal is charged into the capacitor Cst, and the voltage charged into the capacitor Cst is then applied to the gate electrode of the driving transistor M2, such that the driving transistor M2 allows the current to flow. Thus, the OLED emits light.
Here, the current flowing through the OLED provided by the driving transistor M2 is represented by the following equation 1:
                              I          OLED                =                                            β              2                        ⁢                                          (                                  Vgs                  -                                                          Vth                                                                      )                            2                                =                                    β              2                        ⁢                                          (                                  Vdd                  -                  Vdata                  -                  Vth                                )                            2                                                          [                  Equation          ⁢                                          ⁢          1                ]            where, IOLED is a current flowing through the OLED, Vgs is a voltage between the source and the gate of the driving transistor M2, Vth is a threshold voltage of the driving transistor M2, Vdata is a data signal voltage, and β is a gain factor of the driving transistor M2.
From the equation 1, it can be seen that the current IOLED flowing through the OLED varies with the threshold voltage of the driving transistor M2.
However, an organic light emitting diode display has a problem in that deviation of threshold voltages of driving transistors can arise in a manufacturing process, and thus, brightness varies due to a non-uniform amount of currents flowing through OLEDs caused by the deviation of the threshold voltages of the driving transistors (e.g., the driving transistor M2).